1. Field of the Invention
The present invention relates to transmission power control apparatuses and radio communication apparatuses used for digital cellular mobile communications, etc.
2. Description of the Related Art
A multiple access system means a line access system when a plurality of stations carry out communications simultaneously using a same band. For example, CDMA (Code Division Multiple Access) refers to code division multiple access, a technology realizing multiple access through spread spectrum communications transmitting an information signal with its spectrum spread over a sufficiently wide band relative to the original information bandwidth.
This technology is sometimes called xe2x80x9cspread spectrum multiple access (SSMA).xe2x80x9d The mainstream of this CDMA system is a direct spreading system in which a spreading system code is carried on an information signal as is when it is spread.
Since a plurality of communications share a same frequency in the direct spreading CDMA system, the system has a problem (near-far problem) of equalizing the intensity of an interference wave (communication wave of a different station) with that of a desired radio wave at a receiving end and how to overcome this problem is a prerequisite to implement the CDMA system. The perspective problem becomes critical when a base station receives radio waves from many stations at different locations simultaneously, and thus it is essential for a mobile station to control transmission power according to the state of each transmission path.
As a transmission power control method, two methods are available; open-loop control performed based on the reception level of a mobile station and closed-loop control performed by a base station by feeding back information of its reception level to a mobile station as a control signal.
Mobile station transmission power Ti at transmission timing i (i=0 1, . . . ) in open-loop control is expressed as shown in expression (1) using transmission power PBS of the base station and reception level target value Rtg of the base station. In expression (1), Ri stands for the reception power of a desired radio wave.
Ti=Rtg+(PBSxe2x88x92Ri)xe2x80x83xe2x80x83(1)
A CDMA/TDD system which applies a TDD (Time Division Duplex) system that carries out communications by time-dividing a same radio frequency between transmission and reception to the CDMA system, is known to have the ability to implement dynamic and high-precision transmission power control relatively easily by means of open-loop control taking advantage of the high correlativity in characteristics of the propagation path between transmission and reception.
FIG. 1 is a block diagram showing the configuration of transmission power control apparatus 1 that performs open-loop control in conventional CDMA transmission. In this apparatus, reception power calculation circuit 11 calculates the reception power of a desired radio wave using the correlator output. Here, the reception system is provided with an AGC circuit, etc. and if the apparatus has a configuration in which the level of a received signal is adjusted before correlation operations, reception power calculation circuit 11 calculates correct reception power using not only the correlator output but also the adjustment value (AGC gain) above.
The correlator output is input to demodulation circuit 12, and a control signal included in the received signal for controlling transmission power is demodulated and sent to transmission power calculation circuit 13. Transmission power calculation circuit 13 calculates a transmission power set value (Ti of expression (1)) based on the reception power (Ri of expression (1)), transmission power (PBS of expression (1)) and desired reception level (Rtg of expression (1)) of the communication counterpart and outputs it.
On the other hand, transmission power Ti in closed-loop transmission power control is expressed as shown in expression (2) using transmitted power Tixe2x88x921 in the immediately preceding control cycle.
Ti=Tixe2x88x921+Uixe2x80x83xe2x80x83(2)
More precisely, Ui in expression (2) means a variation of power controlled by the control signal and generally indicates a preset value of power variation (hereinafter referred to as xe2x80x9cstepxe2x80x9d) by which the transmission power is increased/decreased in response to the control signal which is an instruction for increasing/decreasing the transmission power. In further explanations, Ui will mean the value described above.
FIG. 2 shows transmission/reception intervals of a mobile station in a communication system which uses the TDD system as the communication system, its cycle (TDD cycle), MS reception power R on the downlink at that time, control signal (TPC for uplink) U and an example of the timing of MS transmission power T.
In FIG. 2, transmission power Ti of MS in TDD cycle i is calculated from expression (1) based on average reception power Ri in the immediately preceding reception interval, known base station transmission power PBS and desired reception level Rtg of the base station in open-loop control, while in closed-loop control it is calculated from expression (2) using received control signal Ui. Thus, in the CDMA/TDD transmission system using the transmission power control apparatus for mobile stations, power received from the base station is controlled so that it may be always fixed at a certain level for all mobile stations.
However, as is clear from expression (1), in open-loop control, the conventional transmission power control apparatus above has a problem that transmission power PBS of the base station and desired reception level Rtg of the base station must be known in order to calculate transmission power Ti from reception power Ri. It also has another problem that it is difficult to perform transmission power control on the downlink.
On the other hand, in closed-loop transmission power control, it has a problem that the transmission speed of the control signal transmitted from the base station to mobile stations increases in order to implement high precision transmission power control according to fading, which will reduce the frequency utilization efficiency.
The present invention has been implemented taking into account the points described above and it is an objective of the present invention to provide a transmission power control apparatus and radio communication apparatus in the CDMA radio system that allow dynamic transmission power control which is a feature of open-loop control without requiring information of the communication counterpart such as Rtg and PBS above, and also realize transmission power control at an equivalent control speed on the reverse line.
The present inventor et al. came to implement the present invention after taking notice of the fact that in the transmission power control apparatus, information on the reception power need not be transmitted from the communication counterpart on the uplink and downlink, discovering that the transmission power can be controlled accurately by storing the reception power of the preceding communication and calculating the transmission power from a difference from the actual reception power of communication using said information, without using the received transmission power PBS of the base station and desired reception level Rtg of the base station.
In other words, the main point of the present invention is to provide a transmission power control apparatus installed on the transmitting side comprising means for calculating desired radio wave reception power from a received signal, means for storing the reception power, means for storing past transmission power, means for demodulating a control signal included in the received signal and means for determining a transmission power set value, and determine the transmission power set value above using the past transmission power, desired radio wave reception power and control signal.
Furthermore, in addition to the above means, said transmission power control apparatus also comprises means for storing a control signal transmitted for transmission power control carried out on the reverse line and can determine the transmission power set value using this control signal, too.
Furthermore, the transmission power control apparatus installed on the receiving side comprises means for calculating the reception power of a desired radio wave or SIR from the received signal, means for comparing with a target value and means for outputting a control signal, measures average reception power of the desired radio wave or SIR from the received signal transmitted by transmission power control, detects a difference from the target value and transmits a control signal based on the result.
Here, in view of performing open-loop transmission power control on the uplink as well as closed-loop transmission power control on the downlink (changing base station transmission power PBS), the control speed (cycle and amount of control) of the downlink must be sufficiently slow (long control cycle or small amount of control) relative to the control speed of control signal Ui that corrects the uplink.
This is because when the difference in the base station reception power caused by a PBS variation is corrected by Ui, performing high-speed control of PBS causes a problem of increasing the transmission speed of control signal Ui that is transferred from the base station to the mobile station, which reduces the frequency utilization efficiency.
By the way, transmission power control of the downlink is often performed for the purpose of keeping constant the communication quality of each mobile station in the system rather than solving perspective problems as in the case of the uplink and closed-loop control is generally performed by feeding back a control signal from the mobile station to the base station based on the reception level or reception SIR information at the mobile station. Therefore, when open-loop transmission power control is performed on the uplink, it is difficult to introduce transmission power control on the downlink with a control speed equivalent to that of the uplink.
Furthermore, the open-loop transmission power control apparatus has a problem that the base station reception power varies from one mobile station to another caused by differences in control of mobile stations. Possible causes are reception power measurements by the AGC circuit and differences produced in the actual transmission power, etc. due to a temperature characteristic with respect to the set values by the PA circuit. Moreover, it is impossible to adaptively control the transmission power of mobile stations to a minimum necessary value according to traffic variations as in the case of closed-loop control.
Therefore, the present inventor et al. invented a corrective method using control signal Ui which is received periodically as shown in expression (3) when closed-loop control is combined. The content of this is also included herein.                     Ti        =                  Rtg          +                      (                                          P                BS                            -              Ri                        )                    +                                    ∑                              m                =                0                            i                        ⁢            Um                                              (        3        )            
This allows the transmission power of the mobile station to be controlled adaptively according to traffic variations as in the case of closed-loop control.
That is, the present inventor et al. solved not only the problems described above but also problems that possibly occur when performing open-loop transmission power control on the uplink and closed-loop transmission power control on the downlink.